A particularly well-rounded set of thoughts on keywording in Lightroom from Chuq Von Rospach here:

An important point  — when you design a structure like this, how well it’ll work for you depends on where your hassle factor hits. Are you more annoyed by looking through long lists of things? If so, design a hierarchy with more sub items and fewer items in each sub-area. If traversing the tree bothers you more, use longer lists and fewer sub-items. You can adjust this to your tastes as you work with it.

[...]

Define some standards and then stick to them. Capitalization, tense, punctuation and the like matters. If you aren’t consistent here, your work will come across as unprofessional and sloppy — even if people looking at your photos only notice it sub-consciously. In my keywords, I standardized on using the plural form (“Birds” instead of “Bird”) unless that was clearly inappropriate, and I lean towards a third person form, present tense and I always strive to use an active voice instead of passive. (Passive Voice writing is to be hated in all serious writing).

Getting all of the details right is — frankly — a pain. But once you get them right, they’ll stay that way with minimal work, and it gives a polish and professionalism to your work that leaves a better impression.

It’s worth reading Chuq’s thoughts in full because they are well-written and include some “theoretical” angles while not forgetting the practical, real world issues. The first paragraph I’ve quoted rings particularly true here – it’s why I use one long list.